An exhaust gas recirculation system (EGR system) has been well known. The EGR system is provided with an exhaust gas cooler (EGR cooler) cooling the recirculated exhaust gas with engine coolant. A combustion temperature is decreased without deteriorating an output of an internal combustion engine, so that noxious agents, such as NOx, contained in the exhaust gas are reduced.
Further, when the engine is at starting state or when an engine coolant temperature is very low in winter, the exhaust gas bypasses the EGR cooler to improve a combustion state of the engine. FIG. 6 shows a conventional EGR system described in WO-2006-084867A1. The EGR system is provided with an EGR cooler 101 cooling an exhaust gas recirculating from an exhaust passage to an intake passage. The recirculated exhaust gas is referred to as an EGR gas. The EGR system is further provided with an exhaust gas switching valve 102 which switches between a cooler-mode and a bypass-mode in order to reduce emission and stabilize a combustion state of the engine. The EGR cooler 101 includes first passages 111 and second passages 113 which are arranged in parallel. The first passages 111 and the second passages 113 are fluidly connected to each other through an intermediate tank 112.
An exhaust gas switching valve 102 is comprised of a housing 103 and a valve body 104. The housing 103 includes an EGR gas inlet port 121 communicating to the exhaust passage of the engine, a cooler inlet port 122 communicating to an inlet of the EGR cooler 101, a cooler outlet port 123 communicating to an outlet of the EGR cooler 101, and an EGR gas outlet port 124 communicating to the intake passage of the engine. The valve body 104 is rotatably accommodated in the housing 103 to switch a communicating condition between the ports 121, 122, 123, 124. The housing 103 includes a shaft supporting portion which supports a shaft 105 of the valve body 104. The valve body 104 is a cantilever valve.
When it is the cooler mode, as shown in FIG. 6, a first EGR gas passage 131 connecting the EGR gas inlet port 121 and the cooler inlet port 122, and a second EGR gas passage 132 connecting the cooler outlet port 123 and the EGR gas outlet port 124 are defined in the housing 103. The EGR gas flows through the EGR cooler 101, so that the temperature of the EGR gas is decreased. When it is the bypass mode, a bypass passage 133 connecting the EGR gas inlet port 121 and the EGR gas outlet port 124 is defined in the housing 103. The EGR gas bypasses the EGR cooler 101.
In the above conventional exhaust gas switching valve 102, in order to drive the valve body 104 stably at any temperature and absorb an assembly tolerance, there is provided valve clearances between an inner surface of the housing 103 and side surfaces of the valve body 104. When it is the cooler mode, a differential pressure between upstream and downstream of the EGR cooler 101 is applied to the valve body 104. The EGR gas flowing through the first EGR gas passage 131 may leak into the second EGR gas passage 132 through the valve clearances.
If the hot EGR gas leaks into the second EGR gas passage 132, the hot EGR gas is mixed with the cooled EGR gas, so that the temperature of the EGR gas flowing into the intake passage through the EGR gas outlet port 124 is increased. Thus, the cooling efficiency of the EGR gas is decreased and the emission reducing performance is deteriorated. Although it is considerable that the valve clearances are made smaller to restrict a leakage of the hot EGR gas, the valve clearances are not easily made smaller due to tolerances of parts and differences in linear expansion coefficient.
The exhaust gas contains exhaust particulates, such as combustion residua or soot. The exhaust particulates become adhesive deposits and may accumulate on the inner surface of the housing 103. When the exhaust gas switching valve 102 is switched from the cooler mode to the bypass mode, the valve body 104 may be stuck by the adhesive deposit.
If the valve clearance is made smaller, the valve body is easily stuck by the adhesive deposit.